The invention relates to a circuit expedient intended for use in electronic flash units of the type whose operation is controlled by a light integrator.
Typically the circuitry of such electronic flash units includes a storage capacitor energized by a source of electrical energy, and connected in parallel to the storage capacitor the series combination of a flash tube and a first electronic switch. The first electronic switch serves to initiate and terminate the flow of current through the flash lamp. The first electronic switch has two main electrodes and a control electrode and can be rendered non-conductive by applying a reverse-bias voltage across its two main electrodes. The type of circuit in question usually includes furthermore a second electronic switch. The conductivity of the second electronic switch is controlled by an automatic exposure control device. The latter senses the light reflected back towards the flash unit from the objects being illuminated by the flash unit. The intensity of the sensed reflected light is integrated, and when the total amount of sensed light reaches a certain value, the automatic exposure control device triggers the second electronic switch. When that occurs, the second electronic switch effects the discharge of a commutation capacitor. The commutation capacitor is so connected in the circuit that such discharge thereof results in the application of a reverse-bias voltage across the main electrodes of the first electronic switch. The application of such reverse-bias voltage initiates the termination of current flow through the flash tube.
Light-integrator-controlled electronic flash units are designed in such a way that the flash tube of the unit gives off only as much light as is necessary for the proper illumination of the subject. To this end, use is made of a circuit arrangement which senses ambient light and, when the total amount of ambient light sensed over a period of time has reached a predetermined threshold value, interrupts the flow of current to the flash tube. For example, if light is given off by the flash tube of the flash unit, such light will in general fall upon objects located not far from the flash unit. A part of such light will accordingly be reflected back towards the flash unit and effect the activation of the light-sensing and integrating means, after the total amount of thusly sensed light has exceeded a certain value. The activation of the light-sensing and integrating means results in the interruption of current flow to the flash lamp, and the latter is thereby extinguished.
U.S. Pat. No. 3,857,064 discloses a circuit arrangement for controlling the operation of a load element which converts the electric current from an energy source into a flash of light. Means are provided for detecting the light from this load element and generating a signal when a certain total amount of light has been produced. This circuit arrangement includes a first electronic switch having two main electrodes, the switch being connected by means of its two main electrodes in series with the load elwement and an energy source. The electronic switch additionally includes a control electrode. The application of a signal to the control electrode renders the first electronic switch conductive, and the switch causes electrical energy to be furnished to the load element so that the latter may produce the flash of light.
This known circuit arrangement further includes a gas-filled tube which serves as a switch. The tube is comprised of closely spaced main electrodes and a firing electrode. One of the main electrodes of this gas-filled tube is connected with one of the main electrodes of the first electronic switch. A commutation capacitor connects the other main electrode of the gas-filled tube to the junction between the first electronic switch and the load element.
A disadvantage of the configuration of this circuit arrangement involves the fact that the gas-filled tube must be provided with a separate control electrode for effecting tube firing.